1. Field of the Invention
The present invention generally relates to alignment methods, exposure methods, pattern forming methods, and exposure apparatuses.
2. Description of the Related Art
With a recent demand for finer semiconductor device patterns and more efficient processes for manufacturing semiconductor devices, the demand for exposure apparatuses, which are used in manufacturing semiconductor devices, capable of realizing higher resolutions and higher efficiencies has been increasing.
In an exposure apparatus, a circuit pattern provided on an original, such as a reticle or a mask, is transferred to a substrate, such as a wafer or a glass plate, over which a photosensitive material (resist) is applied. In general, it is important in transferring a circuit pattern by exposure that relative positioning, i.e., alignment, between the original and the substrate is performed with high accuracy.
In a known alignment method, an alignment mark is transferred to a substrate by exposure simultaneously with a circuit pattern on the original. Among all exposure shots each including such an alignment mark, several shots are preselected and the positions of alignment marks in those preselected shots are measured with an alignment detection optical system. The results of the measurement are statistically processed and the alignment states of all the shots are calculated. In accordance with the results of the calculation, the substrate is positioned with respect to the original.
In recent years, various techniques for improving the resolution have been developed including immersion exposure and chemical shrinking. In particular, a technique called double patterning (double exposure) that can be realized without making a significant modification to the exposure system is considered to be a promising option for realizing finer circuit patterns.
FIG. 15 is a schematic diagram showing a process of double patterning. In double patterning, (a) first exposure, (b) development, and (c) first etching are performed with a first reticle having a circuit pattern thereon. Prior to the first exposure, a first alignment mark A1 provided in a first layer of a substrate is measured, and alignment of the substrate is performed for each shot. In this manner, the circuit pattern of the first reticle together with a second alignment mark A2 are transferred to a target layer of the substrate. Then, after reapplication of a resist, (d) second exposure, (e) development, and (f) second etching are performed with a second reticle having a pattern thereon different from that of the first reticle. Thus, the patterns on the respective reticles are transferred to the same layer. Prior to the second exposure, the second alignment mark A2 that has been transferred to the target layer is measured, and alignment of the substrate is performed for each shot. It is understood that performance of double patterning improves the resolution of the pattern (see “Manufacturing Challenges in Double Patterning Lithography”, William Arnold et al., Manufacturing Control and Execution (MC)-233, International Symposium on Semiconductor Manufacturing (ISSM) 2006, 15th). Double patterning can be performed with either a negative or positive resist.
In double patterning, errors in alignment of a substrate between the first exposure and the second exposure often affect uniformity in the critical dimension (CD, i.e., the minimum line width). Therefore, it is necessary to align the substrate with high accuracy between the first exposure and the second exposure. In general, an allowable alignment error in double patterning is less than 10% of the CD. For example, in a case where a pattern with a half pitch of 45 nm is formed, the allowable alignment error is 2.6 nm.
However, in some cases where the second alignment mark A2 provided in the target layer is measured immediately before the second exposure, the center of the mark A2 may not be calculated accurately because of the asymmetry of the mark A2 itself. In addition, referring to FIG. 15, the mark A2 may not be measured accurately because of the asymmetry of a bottom anti-reflective coating (BARC) and the resist both included in a second second layer that is applied over the mark A2. If data obtained in such a measurement is used in substrate alignment performed in the second exposure, an alignment error may occur. Consequently, a pattern shift may occur.